Frequency control transmission line



April 20, 1937. F H KROGER 2,077,800

FREQUENCY CONTROL TRANSMISSION LINE Filed Feb. 5, 1955 NVENTOR, FRED H KROGER ATTORNEY.

Patented Apr. 20, 1937 UNITED STATES PATENWI` OFFICE FREQUENCY CONTROL TRANSMISSION LINE Delaware Application February 5, 1935, Serial No. 5,058

7 Claims.

This invention relates to an improved high frequency resonant transmission line.

It is well known that a properly designed concentric transmission line having uniformly distributed inductance and capacity has low losses and may be employed for maintaining to a high degree of constancy the frequency of oscillations generated by an electron discharge device system. The line has the effect of a sharply tuned resonant circuit and therefore its reactance changes rapidly with change in frequency, 'and it is this characteristic which is utilized to keep the frequency of the oscillator constant. The resonant frequency of the line is determined chiefly by the length of the line, and for this reason it is important that the length be kept constant in order to maintain a high degree of frequency stability. In a concentric line, when the inner conductor is suiciently shorter than the outer one, the inner one will determine the electrical length of the line; that is, the projection of the inner conductor upon the outer, i. e., the electrical prole of the inner ensemble, will mainly determine the electrical length of the line. An Aarrangement of this general type is adequately described in United States Patent No. 1,980,158, granted November 6, 1934, to C. W. Hansell, and in an article also by C. W. Hansell,

ventitled Resonant Lines for Frequency Control,

published August 1935, in Electrical Engineering, pages 852 to 856, to which reference is made for a more detailed description.

A disadvantage heretofore experienced in resonant transmission lines has been the change in length of the line and the consequent change in resonant frequency thereof due to changes in temperature.

The present invention overcomes this disadvantage and has for one of its objects to provide an improved resonant transmission line Which has an effective substantially constant length irrespective of temperature changes. In accordance with the invention, this is achieved essentially by providing at the free end of the inner conductor of the concentric resonant transmission line a mechanical arrangement to maintain constant the effective length of the inner conductor. A rod having a very low temperature coefcient aids in effecting this result.

A feature of the invention is the metallic sleeve or cap used over the top of the inner conductor which, in combination with a rod of very low temperature coefficient, effectively maintains the electrical length of the inner conductor constant.

A better understanding of the invention may (Cl. TIS- 44) be had by referring to the accompanying drawing which diagrammatically illustrates, in a single iigure, the cross sectional View of the resonant line embodying the principles of the invention, in connection with a suitable oscillator circuit given by way of example only.

Referring to the drawing, there is shown a low loss concentric resonant tube aifair comprising an outer metallic tube i and an inner metallic tube 2, both connected together at one of their adjacent ends by connection 5. An invar rod 4 made of an alloy of nickel and steel and which has a linear coefficient of expansion approximating zero supports metallic cap or sleeve 3 and maintains same at a constant distance from the bottom 5 of the line I, 2. For the purposes of the invention, the upper horizontal part of cap 3 need not be made of metal so long as the wall or sides of cap or sleeve 3 surrounding the uppermost portion of line 2 is constituted of metal and extend suiciently beyond the free end of 2.

Since the electrostatic capacity between inner conducting tube 2 and cap 3 gives negligible reactance compared to the line reactance at the end, the variations in length of inner conductor 2 with change in temperature will not affect the' electrical constants determining the frequency. In consequence, because of the very low temperature coefficient of rod li, the electrical length of the concentric line, as determined by the overall distance from the bottom 5 to the top` of cap 3 will remain practically constant and the resonant frequency of the line will remain fixed.

It is preferred to design line I, 2 to be a quarter wave length long for reasons of reliability and simplicity, although it will be appreciated, of course, that the line may have other lengths by providing lumped capacity near the free end, for example by means of plates located externally of and adjacent the inner conductor and connected to the inner surface of the outer conductor. For greater frequency stability, line I, 2 may be kept free from vibration by suspending it on flexible springs and maintaining the radio frequency equipment on the outer tube I of the line as an integral unit in the manner described in my copending application, Serial No. 1,489, filed January 12, 1935.

The circuit shown in connection with the concentric line I, 2, ii is one which has been used successfully in practice, and comprises an 0scillator electron discharge device Ii to whose grid l is connected one terminal of a grid leak 8, the other terminal of which is variable ovei` the inner conductor 2. The filament power leads for the legs of cathode 9, which are above ground potential and which are maintained at the same radio frequency potential by by-pass condenser II0, are part of a double coil II for controlling regeneration of oscillator 6. The double coil II serves to bring out the filament leads in known inductive relation to each other. Condenser I2 is a vernier for varying the inductance of coil'II, and condenser I3 is a bypass condenser to ground. A spark discharge gap I4 serves as a protective circuit in case of breakdown of tube 6. The filament circuit tuning may be arranged to give suflicient regeneration at the frequency of line I, 2, but to cause oscillations in device 6 to cease when line I, 2 is short circuited.

A 220 volt, 60 cycle, alternating current source energizes the filament of the oscillator 6 through a transformer I5 and a rectifier I6.

In the output of oscillator 6 is a parallel tuned circuit comprising inductance II and a variable condenser I8 coupling the oscillator to a utilization circuit I9', which, if desired, may be a frequency doubler stage. Condenser I8 is a vernier which provides a convenient mechanical method of varying inductance I1, and condenser I9 is a by-pass to ground.

It will be understood, of course, that the invention is not limited to the precise arrangement of parts shown and may be modified without departing from the spirit and scope of the invention. For example, the top of cap or sleeve 3 may comprise one or more slats of material, or else a circular disc, which is preferred merely for its mechanical convenience. Further, the side walls of the cap may be within the inner tube 2 instead of outside the tube.

'I'he term cap or sleeve recited in the speciflcation and claims is intended to cover any arrangement wherein the side walls thereof are either within or outside the inner tubular conductor of the line.

What is claimed is:

1. A resonant transmission line comprising a concentric line having inner and outer conductors electrically coupled together at one of their adjacent ends, a sleeve at the other end of said inner conductor, said sleeve comprising a metallic surface adjacent the walls of said last end of the inner conductor and extending on both sides of the point of termination of said last end, and means having a low coefficient of expansion supporting said sleeve, said means being iixedly positioned with respect to said coupled end of said inner conductor, whereby the overall length of said inner conductor and sleeve is maintained substantially constant.

2. A resonant transmission line comprising inner and outer concentric conductors electrically and mechanically coupled together at one of their adjacent ends, a sleeve having a metallic surface surrounding the other end of said inner conductor and extending on both sides of the point of termination of said last end, and a rod of very low temperature coefficient supporting said sleeve from the coupled ends of said conductors, whereby the overall length of said inner conductor and sleeve is maintained substantially constant.

3. A concentric transmission line having inner and outer tubular conductors mechanically coupled together at one of their adjacent ends,

a metallic sleeve spaced slightly from and surrounding the other end of said inner conductor, and a rod having a low temperature coeiiicient of expansion comprising an alloy of nickel and steel, said rod being located within said inner conductor and mounted at the coupled end of said inner conductor and supporting said sleeve, whereby the overall length of said inner conductor and sleeve is maintained substantially constant.

4. A resonant transmission line having vertically arranged inner and outer concentric tubular conductors conductively coupled together at their lower ends, a sleeve at the upper end of said inner conductor and having metallic walls extending below and above the last end of said inner conductor, and a rod of very low temperature coeflicient located within and extending through the length of the inner conductor for supporting said sleeve, said rod being so arranged that the overall length of the inner conductor and sleeve is maintained substantially constant despite changes in temperature of the atmosphere.

5. A resonant line comprising inner and outer spaced conductors, said inner conductor having at least one free end, and a sleeve comprising a metallic surface adjacent the walls of said inner conductor and extending on both sides of the point of termination of said free end for maintaining the overall length of said inner conductor and sleeve substantially constant despite changes in length of said inner conductor.

6. A resonant line comprising inner and outer hollow concentrically arranged conductors, a metallic end plate mechanically and electrically coupling said conductors together at one of their adjacent ends, a sleeve comprising a metallic surface adjacent the walls of said inner conductor and extending on both sides of the point of termination of the other end of said inner conductor, said sleeve, by virtue of its spacing from said inner conductor, being capacitively coupled to said inner conductor, and a rod of low temperature coeiiicient of expansion located within said inner conductor, said rod being supported at one end from said end plate, the other end of said rod being connected to said sleeve, whereby the overall length of said inner conductor and sleeve is maintained substantially constant despite changes in the ambient temperature.

7. A resonant line comprising inner and outer hollow concentrically arranged conductors, means for xedly coupling said conductors together at one of their adjacent ends, a sleeve comprising a metallic surface adjacent the walls of said inner conductor and extending on both sides of the point of termination of the other end of said inner conductor, said sleeve, by virtue of its spacing from said inner conductor, being capacitively coupled to said inner conductor, and a rod of low temperature coeflicient of expansion located within said inner conductor, said rod being supported at one end from said means, the other end of said rod being connected to said sleeve, whereby the overall length of said inner conductor and sleeve is maintained substantially constant despite changes in the ambient temperature.

FRED H. KROGER. 

